Temperature Distribution And The Effect Of Microstructure In The Heat Affected Zone Of Coiled Tubing

Coiled tubing is a new kind of oil pipe, which is widely used in drilling, well completion, oil production and workover and solve many conventional technique problems. The application effect is obvious. The tube-tube butt welding technology is one part of coiled tubing operation technique system. The microstructure and properties in the heat affected zone (Heat Affect Zone, and HAZ) are uneven discontinuous, which is caused by the welding heat source of butt welding. There will have a softened zone in the HAZ of high grade coiled tubing, which can cause the strength and elongation decline sharply and destroy the overall properties of coiled tubing, greatly reduce the service life and limit the popularization and application of coiled tubing technology.The domestic CT90coiled tubing, with the outer diameter of50.8mm and wall-thickness of4.44mm, was studied in this paper. On the basis of predecessors’research, with proven welding processes and welding methods, the microstructure and properties of welding joints in HAZ with different welding line energy was researched, trying to establish a relationship between welding line energy, HAZ temperature curve and HAZ microstructure, understanding the relationship between microstructure and properties of different processes welding joints and giving a guidance for the welding process in theory. The tube to tube butt welding was used by full position automatic TIG welding, the thermal cycle in HAZ was measured and instantaneous temperature field of coiled tubing was simulated by Ansys finite element software. Through the microscope and scanning electron microscope, the microstructure in HAZ of different welding process was observed and micro hardness tested. Through analysis, the following conclusions were obtained.(1) The temperature in the initiate and extinction arc position will be affected by secondary thermal effect of butt welding heat and the temperature will fluctuate ups and downs;(2) When the welding line energy is small, the heat accumulation effect of coiled tubing butt welding is more obvious, the difference of temperature between the different girth weld circumferential position is great, it takes more time to reach quasi steady state welding temperature field;(3) Under the same welding line energy, the morphology of ring joint in different locations was affected by heat accumulation effect, the molten pool size increases gradually with the welding process;(4) The heat affectedzone of CT90contains coarse grain zone, complete recrystallization zone, incompleterecrystallization zone and tempered zone. The majority of microstructure in coarse grain zoneand complete recrystallization zone is bainite. The majority of microstructure in incompleterecrystallization zone and tempered zone are polygonal ferrite, bainite and carbideprecipitation.(5) The grain size, width and extent of softening zone in heat affected zone isincreased with the heat input.